NASA Earth Observatory (Illustration courtesy Barbara Summey, SSAI) Understanding Stratospheric Ozone Depletion Our understanding of stratospheric ozone depletion has been obtained through a combination of laboratory studies, computer models, and atmospheric observations. The wide variety of chemical reactions that occur in the stratosphere have been discovered and studied in laboratory studies. Chemical reactions between two gases follow well-defined physical rules. Some of these reactions occur on the surfaces of polar stratospheric clouds (PSCs) formed in the winter stratosphere. Reactions have been studied that involve many different molecules containing chlorine, bromine, fluorine, and iodine and other atmospheric constituents such as carbon, oxygen, nitrogen, and hydrogen. These studies have shown that several reactions involving chlorine and bromine directly or indirectly destroy ozone in the stratosphere. Computer models have been used to examine the combined effect of the large group of known reactions that occur in the stratosphere. These models simulate the stratosphere by including representative chemical abundances, winds, air temperatures, and the daily and seasonal changes in sunlight. These analyses show that under certain conditions chlorine and bromine react in catalytic cycles in which one chlorine or bromine atom destroys many thousands of ozone molecules.

Models are also used to simulate ozone amounts observed in previous years as a strong test of our understanding of atmospheric processes and to evaluate the importance of new reactions found in laboratory studies. The responses of ozone to possible future changes in the abundances of trace gases, temperatures, and other atmospheric parameters have been extensively explored with specialized computer models . Atmospheric observations have shown what gases are present in different regions of the stratosphere and how their abundances vary. Gas and particle abundances have been monitored over time periods spanning a daily cycle to decades. Observations show that halogen source gases and reactive halogen gases are present in the stratosphere at the amounts required to cause observed ozone depletion. Ozone and chlorine monoxide (ClO), for example, have been observed extensively with a variety of instruments. ClO is a highly reactive gas that is involved in catalytic ozone destruction cycles throughout the stratosphere . Instruments on the ground and on satellites, balloons, and aircraft now routinely detect ozone and ClO remotely using optical and microwave signals. High-altitude aircraft and balloon instruments are also used to detect both gases locally in the stratosphere .

The observations of ozone and reactive gases made in past decades are used extensively in comparisons with computer models in order to increase confidence in our understanding of stratospheric ozone depletion. The stratospheric ozone layer shields life on Earth from the Sun’s harmful ultraviolet radiation. Chemicals that destroy ozone are formed by industrial and natural processes. With the exception of volcanic injection and aircraft exhaust, these chemicals are carried up into the stratosphere by strong upward-moving air currents in the tropics. Methane (CH4), chlorofluorocarbons (CFCs), nitrous oxide (N2O) and water are injected into the stratosphere through towering tropical cumulus clouds. These compounds are broken down by the ultraviolet radiation in the stratosphere. Byproducts of the breakdown of these chemicals form “radicals”—such as nitrogen dioxide (NO2) and chlorine monoxide (ClO)—that play an active role in ozone destruction. Aerosols and clouds can accelerate ozone loss through reactions on cloud surfaces. Thus, volcanic clouds and polar stratospheric clouds can indirectly contribute to ozone loss. Stratospheric air temperatures in both polar regions reach minimum values in the lower stratosphere in the winter season.

Average minimum values over Antarctica are as low as –90°C in July and August in a typical year. Over the Arctic, average minimum values are near –80°C in January and February. Polar stratospheric clouds (PSCs) are formed when winter minimum temperatures fall below the formation temperature (about –78°C). This occurs on average for 1 to 2 months over the Arctic and 5 to 6 months over Antarctica (see heavy red and blue lines). Reactions on PSCs cause the highly reactive chlorine gas ClO to be formed, which increases the destruction of ozone. The range of winter minimum temperatures found in the Arctic is much greater than in the Antarctic. In some years, PSC formation temperatures are not reached in the Arctic, and significant ozone depletion does not occur. In the Antarctic, PSCs are present for many months, and severe ozone depletion now occurs in each winter season. The animation illustrates how one chlorine atom in the stratosphere can destroy up to 100,000 ozone molecules. Credit University Of Alaska Ozone is destroyed by reactions with chlorine, bromine, nitrogen, hydrogen, and oxygen gases. Reactions with these gases typically occurs through catalytic processes.

A catalytic reaction cycle is a set of chemical reactions which result in the destruction of many ozone molecules while the molecule that started the reaction is reformed to continue the process. Because of catalytic reactions, an individual chlorine atom can on average destroy nearly a thousand ozone molecules before it is converted into a form harmless to ozone. Emission, accumulation, and transport. The process begins with the emission, at Earth’s surface, of source gases containing the halogens chlorine and bromine . The halogen source gases, often referred to as ozone-depleting substances (ODSs), include manufactured chemicals released to the atmosphere in a variety of applications, such as refrigeration, air conditioning, and foam blowing. Chlorofluorocarbons (CFCs) are an important example of chlorine-containing gases. Emitted source gases accumulate in the lower atmosphere (troposphere) and are transported to the stratosphere by natural air motions. The accumulation occurs because most source gases are highly unreactive in the lower atmosphere. Small amounts of these gases dissolve in ocean waters. The low reactivity of these manufactured halogenated gases is one property that makes them well suited for specialized applications such as refrigeration.

Some halogen gases are emitted in substantial quantities from natural sources . These emissions also accumulate in the troposphere, are transported to the stratosphere, and participate in ozone destruction reactions. These naturally emitted gases are part of the natural balance of ozone production and destruction that predates the large release of manufactured halogenated gases. Conversion, reaction, and removal. Halogen source gases do not react directly with ozone. Once in the stratosphere, halogen source gases are chemically converted to reactive halogen gases by ultraviolet radiation from the Sun . The rate of conversion is related to the atmospheric lifetime of a gas ). Gases with longer lifetimes have slower conversion rates and survive longer in the atmosphere after emission. Lifetimes of the principal ODSs vary from 1 to 100 years . Emitted gas molecules with atmospheric lifetimes greater than a few years circulate between the troposphere and stratosphere multiple times, on average, before conversion occurs. The reactive gases formed from halogen source gases react chemically to destroy ozone in the stratosphere . The average depletion of total ozone attributed to reactive gases is smallest in the tropics and largest at high latitudes .

In polar regions, surface reactions that occur at low temperatures on polar stratospheric clouds (PSCs) greatly increase the abundance of the most reactive chlorine gas, chlorine monoxide (ClO) . This results in substantial ozone destruction in polar regions in late winter and early spring . After a few years, air in the stratosphere returns to the troposphere, bringing along reactive halogen gases. These gases are then removed from the atmosphere by rain and other precipitation or deposited on Earth’s land or ocean surfaces. This removal brings to an end the destruction of ozone by chlorine and bromine atoms that were first released to the atmosphere as components of halogen source gas molecules. Tropospheric conversion. Halogen source gases with short lifetimes (less than 1 year) undergo significant chemical conversion in the troposphere, producing reactive halogen gases and other compounds. Source gas molecules that are not converted are transported to the stratosphere. Only small portions of reactive halogen gases produced in the troposphere are transported to the stratosphere because most are removed by precipitation.

Important examples of halogen gases that undergo some tropospheric removal are the hydrochlorofluorocarbons (HCFCs), methyl bromide (CH3Br), and gases containing iodine . Diagram showing the effect of polar stratospheric clouds on ozone loss. The upper panel shows the situation when there are no polar stratospheric clouds. Ozone depletion takes place only in the gas phase (homogeneous chemistry). The lower panel shows the situation when there are polar stratospheric clouds present. The reservoir gases hydrochloric acid and chlorine nitrate react with each other on the surface of the PSC particles through a red-ox reaction and liberate elementary chlorine (Cl2 ). Elementary chlorine is easily photolysed by sunlight and forms atomic chlorine, which reacts fast with ozone to form chlorine monoxide (ClO, active chlorine) and oxygen (O2 ). ClO dimerises and forms Cl2O2, which is easily photolysed, liberating atomic chlorine again. Due to this catalytic cycle, one atom of Cl can destroy thousands of ozone molecules before it is passivated through reaction with NO2, methane or other substances. This explains why a few ppb of chlorine can destroy several ppm of ozone.

In addition, PSC particles can grow large enough to sediment, thereby removing HNO3 from the stratosphere. This means that there will be limited amounts of NO2 present to quench the active chlorine, and the ozone depleting process can continue for several weeks. The diagram has been made by Finn Bjørklid, Norwegian Institute for Air Research (NILU). Stratospheric ozone depletion milestones. This timeline highlights milestones related to the history of ozone depletion. Events represent the occurrence of important scientific findings, the completion of international scientific assessments, and highlights of the Montreal Protocol. The graph shows the history and near future of annual total emissions of ozone-depleting substances (ODSs) combined with natural emissions of halogen source gases. ODSs are halogen source gases controlled under the Montreal Protocol. The emissions, when weighted by their potential to destroy ozone, peaked near 1990 after several decades of steady increases (see Q19). Between 1990 and the present, emissions have decreased substantially as a result of the Montreal Protocol and its subsequent Amendments and Adjustments coming into force . The Protocol began with the Vienna Convention for the Protection of the Ozone Layer in 1985.

The provisions of the Protocol and its Amendments and Adjustments decisions have depended on information embodied in international scientific assessments of ozone depletion that have been produced periodically since 1989 under the auspices of UNEP and WMO. Atmospheric observations of ozone, CFCs, and other ODSs have increased substantially since the early 1970s. For example, the SAGE and TOMS satellite instruments have provided essential global views of stratospheric ozone for several decades. The Nobel Prize in Chemistry in 1995 was awarded for research that identified the threat to ozone posed by CFCs and that described key reactive processes in the stratosphere. By 2008, stratospheric chlorine abundances in the stratosphere were 10% lower than their peak values reached in the late 1990s and were continuing to decrease. January 2010 marked the end of global production of CFCs and halons under the Protocol.

(A megatonne = 1 billion (109) kilograms.) Ozone Production and Destruction — GES DISC - Goddard Earth Sciences Data and Information Services Center Info Ozone Production and Destruction Stratospheric ozone is created and destroyed primarily by ultraviolet (UV) radiation. The air in the stratosphere is bombarded continuously with UV radiation from the Sun.When high energy UV rays strike molecules of ordinary oxygen (O2), they split the molecule into two single oxygen atoms.The free oxygen atoms can then combine with oxygen molecules (O2) to form ozone (O3) molecules. O2 + UV light → 2 O O + O2 + M → O3 + M (where M indicates conservation of energy and momentum) The same characteristic of ozone that makes it so valuable – its ability to absorb a range of UV radiation – also causes its destruction. When an ozone molecule is exposed to UV energy it may revert back into O2 and O. During dissociation, the atomic and molecular oxygens gain kinetic energy, which produces heat and causes an increase in atmospheric temperature. Ozone production is driven by UV radiation of wavelengths less than 240 nm.

Ozone dissociation typically produces atomic oxygen (O) that is stable when exposed to longer UV wavelengths, up to 320 nm, and visible light wavelengths of 400-700 nm. Longer wavelength photons can penetrate deeper into the atmosphere, creating regions of ozone production and destruction. When an ozone molecule absorbs even low energy UV, it splits into an ordinary oxygen molecule and a free oxygen atom. O3 + UV, visible light → O + O2 The free oxygen atom may then combine with an oxygen molecule, creating another ozone molecule, or it may take an oxygen atom from an existing ozone molecule to create two ordinary oxygen molecules. O + O2 → O3 or O3 + O → O2 + O2 Processes of ozone production and destruction, initiated by ultraviolet radiation, are often referred to as Chapman Reactions. Most O3 destruction takes place through catalytic processes rather than Chapman Reactions.Ozone is a highly unstable molecule that readily donates its extra oxygen molecule to free radical specie,s such as nitrogen, hydrogen, bromine, and chlorine.These compounds naturally occur in the stratosphere, released from sources such as soil, water vapor, and the oceans.

O3 + X → XO + O2 ( where X may be O, NO, OH, Br or Cl) Anthropogenic Destruction Manufactured compounds are also capable of altering atmospheric ozone levels. Chlorine, released from CFCs, (15k JPG) and bromine (Br), released from halons, are two of the most important chemicals associated with ozone depletion. Halons are primarily used in fire extinguishers. CFCs are used extensively in aerosols, air conditioners, refrigerators, and cleaning solvents. Two major types of CFCs are trichlorofluorocarbon (CFC13), or CFC-11,and dichlorodifluoromethane (CF2Cl2), or CFC-12. Trichlorofluorocarbon is used in aerosols, while dichlorodifluoromethane is typically used as a coolant. CFCs were originally created to provide a substitute for toxic refrigerant gases and to reduce the occupational hazard of compressor explosions. Near Earth's surface, chloroflourocarbons are relatively harmless and do not react with any material, including human skin.

For 50 years they appeared to be the perfect example of a benign technical solution to environmental and engineering problems, with no negative side effects. While CFCs remain in the troposphere, they are virtually indestructible. They are not water soluble and cannot even be washed out of the atmosphere by rain. We now understand that the very quality that made them seem so safe –their stability – is what makes them so dangerous to the Earth system. CFCs remain in the troposphere for more than 40 years before their slow migration to the stratosphere is complete. Even if we were to end their production and use at this very moment, they will continue to contribute to ozone destruction far into the future. In the stratosphere, high energy UV radiation causes the CFC molecules to break down through photodissociation. Atomic chlorine, a true catalyst for ozone destruction, is released in the process. Chlorine initiates and takes part in a series of ozone-destroying chemical reactions and emerges from the process unchanged.

The free chlorine atom initially reacts with an unstable oxygen containing compound (such as ozone) to form chlorine monoxide (ClO): Cl + O3 → ClO + O2 The ClO molecule then reacts with atomic oxygen to produce molecular oxygen (O2) and more atomic chlorine. The regenerated Cl atom is then free to initiate a new cycle: ClO + O → Cl + O2 This destructive chain of reactions will continue over and over again, limited only by the amount of chlorine available to fuel the process. Chlorine occurs naturally in the oceans. However, the majority of chlorine in the atmosphere has originated with man-made chemicals. Without the dissociation of manufactured chlorofluorocarbons, there would be almost no chlorine in the stratosphere. CFC-12 concentrations were less than 100 parts per trillion by volume when they were first measured in the 1960s. Between 1975 and 1987, concentrations more than doubled from less than 200 parts per trillion by volume to more than 400 parts per trillion by volume. The amount of chlorine in the stratosphere also increased by a factor of 2 to 3.

Scientists believe that continued buildup of CFCs could lead to severe ozone loss (61k JPG) worldwide. Thus, ongoing studies are essential to provide a necessary understanding of the causes of ozone depletion.The history of CFCs demonstrates that human activities can have unexpected long-term effects on the environment. CFCs Chlorofluorocarbon (CFC) is an organic compound that contains carbon, chlorine, and fluorine, produced as a volatile derivative of methane and ethane. A common subclass is the hydrochlorofluorocarbons (HCFCs), which contain hydrogen, as well. Freon is DuPont's brand name for CFCs, HCFCs and related compounds. Other commercial names from around the world are Algofrene, Arcton, Asahiflon, Daiflon, Eskimo, FCC, Flon, Flugene, Forane, Fridohna, Frigen, Frigedohn, Genetron, Isceon, Isotron, Kaiser, Kaltron, Khladon, Ledon, Racon, and Ucon. The most common representative is dichlorodifluoromethane (R-12 or Freon-12).

Chlorofluorocarbons (CFCs) are a family of chemical compounds developed back in the 1930's as safe, non-toxic, non-flammable alternative to dangerous substances like ammonia for purposes of refrigeration and spray can propellants. Their usage grew enormously over the years. One of the elements that make up CFCs is chlorine. Very little chlorine exists naturally in the atmosphere. But it turns out that CFCs are an excellent way of introducing chlorine into the ozone layer. The ultraviolet radiation at this altitude breaks down CFCs, freeing the chlorine. Under the proper conditions, this chlorine has the potential to destroy large amounts of ozone. This has indeed been observed, especially over Antarctica. As a consequence, levels of genetically harmful ultraviolet radiation have increased. Production of new stocks ceased in most countries as of 1994 However many countries still require aircraft to be fitted with halon fire suppression systems because no safe and completely satisfactory alternative has been discovered for this application. There are also a few other, highly specialized uses.

These programs recycle halon through "halon banks" coordinated by the Halon Recycling Corporation to ensure that discharge to the atmosphere occurs only in a genuine emergency and to conserve remaining stocks. Development of alternatives for CFCs Work on alternatives for chlorofluorocarbons in refrigerants began in the late 1970s after the first warnings of damage to stratospheric ozone were published. The hydrochlorofluorocarbons (HCFCs) are less stable in the lower atmosphere, enabling them to break down before reaching the ozone layer. Nevertheless, a significant fraction of the HCFCs do break down in the stratosphere and they have contributed to more chlorine buildup there than originally predicted. Later alternatives lacking the chlorine, the hydrofluorocarbons (HFCs) have an even shorter lifetimes in the lower atmosphere. One of these compounds, HFC-134a, is now used in place of CFC-12 in automobile air conditioners. Hydrocarbon refrigerants (a propane/isobutane blend) are also used extensively in mobile air conditioning systems in Australia, the USA and many other countries, as they have excellent thermodynamic properties and perform particularly well in high ambient temperatures.

One of the natural refrigerants (along with Ammonia and Carbon Dioxide), hydrocarbons have negligible environmental impacts and are also used worldwide in domestic and commercial refrigeration applications, and are becoming available in new split system air conditioners Applications and replacements for CFCs Application CFC History Refrigerators from the late 1800s until 1929 used the toxic gases, ammonia (NH3), methyl chloride (CH3Cl), and sulfur dioxide (SO2), as refrigerants. Several fatal accidents occurred in the 1920s because of methyl chloride leakage from refrigerators. People started leaving their refrigerators in their backyards. A collaborative effort began between three American corporations, Frigidaire, General Motors and DuPont to search for a less dangerous method of refrigeration. In 1928, Thomas Midgley, Jr. aided by Charles Franklin Kettering invented a "miracle compound" called Freon. Freon represents several different chlorofluorocarbons, or CFCs, which are used in commerce and industry.

The CFCs are a group of aliphatic organic compounds containing the elements carbon and fluorine, and, in many cases, other halogens (especially chlorine) and hydrogen. Freons are colorless, odorless, nonflammable, noncorrosive gases or liquids. Chlorofluorocarbons (CFCs) are highly stable compounds that were used as propellents in spray cans and in refrigeration units. They are several organic compounds composed of carbon, fluorine, chlorine, and hydrogen. CFCs are manufactured under the trade name Freon . The invention of chlorofluorocarbons (CFCs) in the late 1920s and early 1930s stemmed from the call for safer alternatives to the sulfur dioxide and ammonia refrigerants used at the time, CFCs found wide application after World War II. Chloroflourocarbons were first created in 1928 as non-toxic, non-flamable refrigerants, and were first produced commercially in the 1930's by DuPont.

The first Chlorofluorocarbon was CFC-12, a single carbon with two chlorines and two Fluorines attached to it. These halogenated hydrocarbons, notably trichlorofluoromethane (CFC-11, or F-11) and dichlorodifluoromethane (CFC-12, or F-12), have been used extensively as aerosol-spray propellants, refrigerants, solvents, and foam-blowing agents. They are well-suited for these and other applications because they are nontoxic and nonflammable and can be readily converted from a liquid to a gas and vice versa. Chlorofluorocarbons or CFCs (also known as Freon) are non-toxic, non-flammable and non-carcinogenic. They contain fluorine atoms, carbon atoms and chlorine atoms.

The 5 main CFCs include CFC-11 (trichlorofluoromethane - CFCl3), CFC-12 (dichloro-difluoromethane - CF2Cl2), CFC-113 (trichloro-trifluoroethane - C2F3Cl3), CFC-114 (dichloro-tetrfluoroethane - C2F4Cl2), and CFC-115 (chloropentafluoroethane - C2F5Cl). CFCs have been found to pose a serious environmental threat. Studies undertaken by various scientists during the 1970s revealed that CFCs released into the atmosphere accumulate in the stratosphere, where they had a deleterious effect on the ozone layer. Stratospheric ozone shields living organisms on Earth from the harmful effects of the Sun's ultraviolet radiation; even a relatively small decrease in the stratospheric ozone concentration can result in an increased incidence of skin cancer in humans and in genetic damage in many organisms. In the stratosphere the CFC molecules break down by the action of solar ultraviolet radiation and release their constituent chlorine atoms.

These then react with the ozone molecules, resulting in their removal. CFCs have a lifetime in the atmosphere of about 20 to 100 years, and consequently one free chlorine atom from a CFC molecule can do a lot of damage, destroying ozone molecules for a long time. Although emissions of CFCs around the developed world have largely ceased due to international control agreements, the damage to the stratospheric ozone layer will continue well into the 21st century. In 1978 The Montreal Protocol was adopted as a framework for international cooperation regarding CFC control on the basis of the Vienna Convention for the Protection of the Ozone Layer. Data compiled from The British Antarctic Study, NASA, NASA Ozone Watch, Environment Canada, UNEP, EPA and other sources as stated and credited Updated Daily-Researched By Charles Welch- This Website is a project of the The Ozone Hole Inc. Visit our other website Copyright Charles Welch The Ozone Hole Ozone Layer and Causes, Effects and Solutions to Ozone Depletion - Conserve Energy Future Conserve Energy Future Energy Articles What is Ozone Layer? To understand ozone layer, it would be helpful to know the different layers of the atmosphere.

The earth’s atmosphere is composed of many layers, each playing a significant role. The first layer stretching approximately 10 kilometers upwards from the earth’s surface is known as the troposphere. A lot of human activities such as gas balloons, mountain climbing, and small aircraft flights take place within this region. The stratosphere is the next layer above the troposphere stretching approximately 15 to 60 kilometers. The ozone layer sits in the lower region of the stratosphere from about 20-30 kilometers above the surface of the earth. The thickness of the ozone layer is about 3 to 5 mm, but it pretty much fluctuates depending on the season and geography. Ozone layer is a deep layer in earth’s atmosphere that contain ozone which is a naturally occurring molecule containing three oxygen atoms. These ozone molecules form a gaseous layer in the Earth’s upper atmosphere called stratosphere. This lower region of stratosphere containing relatively higher concentration of ozone is called Ozonosphere. The Ozonosphere is found 15-35 km (9 to 22 miles) above the surface of the earth.

The concentration of ozone in the ozone layer is usually under 10 parts per million while the average concentration of ozone in the atmosphere is about 0.3 parts per million . The thickness of the ozone layer differs as per season and geography. The highest concentrations of ozone occur at altitudes from 26 to 28 km (16 to 17 miles) in the tropics and from 12 to 20 km (7 to 12 miles) towards the poles. The ozone layer forms a thick layer in stratosphere, encircling the earth, that has large amount of ozone in it. The ozone layer protects life on earth from strong ultraviolet radiation that comes from the sun. Ultraviolet rays are harmful rays that can drive up the risk of deadly disorders like skin cancer, cataracts and damage the immune system. Ultraviolet rays are also capable of destroying single cell organism, terrestrial plant life, and aquatic ecosystems. The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson.

The ozone layer has the capability to absorb almost 97-99% of the harmful ultraviolet radiations that sun emit and which can produce long term devastating effects on humans beings as well as plants and animals . Composition of the Ozone Layer It comes as a surprise that the same UV rays form the bulk of ozone layer. Ozone is an extraordinary kind of oxygen composed of 3 oxygen atoms instead of the normal 2 oxygen atoms. Ozone layer normally develops when a few kinds of electrical discharge or radiation splits the 2 atoms in an oxygen(O2) molecule, which then independently reunite with other types of molecules to form ozone. The ozone layer has been shielding life on planet earth for billions of years, but it’s now being worn out by human activities. People began to value the importance of the ozone layer when scientists released a research finding suggesting that certain human-made chemicals known as chlorofluorocarbons managed to reach the stratosphere and depleted the ozone via a profound series of chemical reactions. The results of this research study prompted the signing of a global treaty known as the Montreal Protocol in 1973. This treaty helped in the reduction of the production of these harmful human-made chemicals.

These targeted efforts have seen the ozone layer recovering over the past years. The thickness of the ozone layer varies immensely on any day and location. Due to relentless vertical atmospheric air circulation in both the stratosphere and troposphere, the amount of ozone layer shielding humans from strong UV rays can be lesser or greater. In addition, those residing in higher elevations are at risk of UV radiation than those at lower elevations. The Stratospheric ozone plays a big role in protecting humans from the harshness of the sun. However, there is also a kind of ozone developed just above the ground as a result of sun rays coming into contact with pollution in the atmosphere , which is hazardous to human health. In some individuals, it can lead to complications in breathing and often takes place during summer when pollution is rampant in cities where the air is static. Why Ozone Layer is Necessary? An essential property of ozone molecule is its ability to block solar radiations of wavelengths less than 290 nanometers from reaching Earth’s surface. In this process, it also absorbs ultraviolet radiations that are dangerous for most living beings. UV radiation could injure or kill life on Earth.

Though the absorption of UV radiations warms the stratosphere but it is important for life to flourish on planet Earth. Research scientists have anticipated disruption of susceptible terrestrial and aquatic ecosystems due to depletion of ozone layer. Ultraviolet radiation could destroy the organic matter. Plants and plankton cannot thrive, both acts as food for land and sea animals, respectively. For humans, excessive exposure to ultraviolet radiation leads to higher risks of cancer (especially skin cancer) and cataracts. It is calculated that every 1 percent decrease in ozone layer results in a 2-5 percent increase in the occurrence of skin cancer. Other ill-effects of the reduction of protective ozone layer include – increase in the incidence of cataracts, sunburns and suppression of the immune system. Causes of Ozone Layer Depletion Credible scientific studies have substantiated that the cause of ozone layer depletion is human activity, specifically, human-made chemicals that contain chlorine or bromine. These chemicals are widely known as ODS, an acronym for Ozone-Depleting Substances. The scientists have observed reduction in stratospheric ozone since early 1970’s. It is found to be more prominent in Polar Regions.

Ozone-Depleting Substances have been proven to be eco-friendly , very stable and non-toxic in the atmosphere below. This is why they have gained popularity over the years. However, their stability comes at a price; they are able to float and remain static high up in the stratosphere. When up there, ODS are comfortably broken down by the strong UV light and the resultant chemical is chlorine and bromine. Chlorine and bromine are known to deplete the ozone layer at supersonic speeds. They do this by simply stripping off an atom from the ozone molecule. One chlorine molecule has the capability to break down thousands of ozone molecules. Ozone-depleting substances have stayed and will continue to stay in the atmosphere for many years. This, essentially, implies that a lot of the ozone-depleting substances human have allowed to go into the atmosphere for the previous 90 years are still on their journey to the atmosphere, which is why they will contribute to ozone depletion. The chief ozone-depleting substances include chlorofluorocarbons (CFCs), carbon tetrachloride, hydrochlorofluorocarbons (HCFCs) and methyl chloroform.

Halons, sometimes known as brominated fluorocarbons, also contribute mightily to ozone depletion. However, their application is greatly restricted since they are utilized in specific fire extinguishers. The downside to halons is they are so potent that they are able to deplete the ozone layer 10 times more than ozone-depleting substances. Scientists in this age are working around the clock to develop Hydrofluorocarbons (HFCs) to take the place of hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) for use in vehicle air conditioning. Hydrochlorofluorocarbons are powerful greenhouse gases , but they are not able to deplete ozone. Chlorofluorocarbons, on the other hand, significantly contribute to climate change, which means Hydrofluorocarbons continue to be the better alternative until safer alternatives are available. There are two regions in which the ozone layer has depleted. In the mid-latitude, for example, over Australia, ozone layer is thinned. This has led to an increase in the UV radiation reaching the earth.

It is estimated that about 5-9% thickness of the ozone layer has decreased, increasing the risk of humans to over-exposure to UV radiation owing to outdoor lifestyle. In atmospheric regions over Antarctica, ozone layer is significantly thinned, especially in spring season. This has led to the formation of what is called ‘ozone hole’. Ozone holes refer to the regions of severely reduced ozone layers. Usually ozone holes form over the Poles during the onset of spring seasons. One of the largest such hole appears annually over Antarctica between September and November. Natural causes of depletion of ozone layer: Ozone layer has been found to be affected by certain natural phenomena such as Sun-spots and stratospheric winds. But this has been found to cause not more than 1-2% depletion of the ozone layer and the effects are also thought to be only temporary. It is also believed that the major volcanic eruptions (mainly El Chichon in 1983 and and Mt. Pinatubo in 1991) has also contributed towards ozone depletion.

Man-made causes of depletion of ozone layer: The main cause for the depletion of ozone is determined as excessive release of chlorine and bromine from man-made compounds such as chlorofluorocarbons (CFCs). CFCs (chlorofluorocarbons), halons, CH3CCl3 (Methyl chloroform), CCl4 (Carbon tetrachloride), HCFCs (hydro-chlorofluorocarbons), hydrobromofluorocarbons and methyl bromide are found to have direct impact on the depletion of the ozone layer. These are categorized as ozone-depleting substances (ODS). The problem with the Ozone-Depleting Substances (ODS) is that they are not washed back in the form of rain on the earth and in-fact remain in the atmosphere for quite a long time. With so much stability, they are transported into the stratosphere. The emission of ODS account for roughly 90% of total depletion of ozone layer in stratosphere.

These gases are carried to the stratosphere layer of atmosphere where ultraviolet radiations from the sun break them to release chlorine (from CFCs) and bromine (from methyl bromide and halons). The chlorine and bromine free radicals react with ozone molecule and destroy their molecular structure, thus depleting the ozone layer. One chlorine atom can break more than 1, 00,000 molecules of ozone. Bromine atom is believed to be 40 times more destructive than chlorine molecules. Main Ozone Depleting Substances (ODS) Chlorofluorocarbons (CFCs) It’s billed as the most extensively utilized ozone-depleting substance because it attributes to more than 80% of overall ozone depletion. It was utilized as a coolant in home appliances like freezers, refrigerators and air conditioners in both buildings and cars that were manufactured prior to 1995. This substance is usually contained in dry cleaning agents, hospital sterilants, and industrial solvents. The substance is also utilized in foam products like mattresses and cushions and home insulation.

Hydrofluorocarbons (HCFCs) Hydrofluorocarbons have over the years served in place of Chlorofluorocarbons. They are not as harmful as CFCs to ozone layer. Halons It’s especially used in selected fire extinguishers in scenarios where the equipment or material could be devastated by water or extinguisher chemicals. Carbon Tetrachloride Also used in selected fire extinguishers and solvents. Methyl Chloroform Commonly utilized in industries for cold cleaning, vapor degreasing, chemical processing, adhesives and some aerosols. Serious Effects of Ozone Depletion Damage to human health If the ozone layer is depleted, it means humans will be overly exposed to strong UV light. Overexposure to strong UV light causes skin cancer, cataracts, sunburns, weakening of immune system and quick aging. Devastation to environment Many crops species are vulnerable to strong UV light and overexposure may well lead to minimal growth, photosynthesis and flowering. Some of the crop species vulnerable to UV light include barley, wheat, corn, oats, rice, broccoli, tomatoes, cauliflower just to name a few. Forests equally bear the brunt of ozone depletion.

Threat to marine life Certain marine life, especially planktons, is greatly impacted by exposure to strong ultraviolet rays. In the aquatic food chain, planktons appear high up. If planktons decrease in number due to ozone layer destruction, the marine food chain would be disrupted in many ways. Also, overexposure of sun rays could reduce the fortunes of fishers. On top of that, certain species of marine life have been greatly affected by overexposure to ultraviolet radiation at their early stage. Effect on animals In domesticated animals, too much Ultraviolet radiation could also lead to skin and eye cancer. Impacts certain materials Materials like plastics, wood, fabrics, rubber are massively degraded by too much ultraviolet radiation Solutions to Ozone Depletion Desist from using pesticides Pesticides are great chemicals to rid your farm of pests and weeds, but they contribute enormously to ozone layer depletion. The surefire solution to get rid of pests and weeds is to apply natural methods. Just weed your farm manually and use alternative eco-friendly chemicals to alleviate pests. Discourage driving of private vehicles The easiest technique to minimize ozone depletion is to limit the number of vehicles on the road.

These vehicles emit a lot of greenhouse gases that eventually form smog , a catalyst in the depletion of ozone layer. Utilize environmentally friendly cleaning products Most household cleaning products are loaded with harsh chemicals that find way to the atmosphere, eventually contributing to degradation of the ozone layer. Use natural and environmentally friendly cleaning products to arrest this situation. Prohibit the use of harmful nitrous oxide The Montreal Protocol formed in 1989 helped a lot in the limitation of Chlorofluorocarbons (CFCs). However, the protocol never covered nitrous oxide, which is a known harmful chemical that can destroy the ozone layer. Nitrous oxide is still in use today. Governments must take action now and outlaw nitrous oxide use to reduce the rate of ozone depletion. Chlorofluorocarbons Refrigerants, CFC Refrigerants: Ozone Layer Depletion & Greenhouse Effect Chlorofluorocarbons (CFCs): Refrigerants that Cause Ozone Layer Depletion written by: Haresh Khemani • edited by: Swagatam • updated: 1/15/2010 Chlorofluorocarbons (CFCs) have been found to cause the depletion of the ozone layer.

The ozone layer prevents ultraviolet radiations of the sun from coming on the earth. This article describes the process of damaging effects of CFCs. slide 1 of 3 What are CFCs? Chlorofluorocarbons (CFCs) have been used extensively in last five or six decades as refrigerants in the vapor compression cycle to produce refrigerating and air-conditioning effects. In recent years it has been found that CFCs are most destructive to the environment. It has been proved that CFCs are a major cause of depletion of the earth’s stratospheric ozone layer and contribute to the greenhouse effect (global warming). Presently large quantities of CFCs are being used as refrigerants in a number of refrigerating and air-conditioning systems. Though the refrigerant moves in a closed cycle, there are lots of leakages that escape to the atmosphere and cause destruction of the ozone layer. The most shocking fact about CFCs is that they have exceptionally long atmospheric life which, in certain cases, even extends to 100 years. This means that if CFC refrigerants are leaked in the atmosphere, they will keep depleting ozone layer for the next 100 years to come.

slide 2 of 3 How CFCs Destroy Ozone Layer When the CFC refrigerants are leaked from refrigeration or air-conditioning systems, they drift around the lower layers of the atmosphere. Slowly they start infiltrating into the upper layers of the atmosphere and soon reach the ozone rich stratosphere, where they undergo major chemical changes. What Ozone Does: In the ozone layer sunlight enters in its pristine pure form; it is called ultraviolet radiation, which is highly intense and dangerous to plant and human life. The ozone layer filters this highly intense sunlight and allows less intense sunlight, which is not harmful to human and plant life, to the surface of the earth. How it Changes CFCs The unfiltered sunlight bombards the molecules of CFC refrigerants, and they are pushed towards the stratospheric clouds over the poles. Due to this the CFC molecules get disintegrated. The chlorine atom removed from CFCs reacts with ozone molecule (O3) and converts it into oxygen molecule (O2). Ozone is Transformed to Oxygen Now, the oxygen does not have the capacity to filter the highly intense ultraviolet radiations.

So what is happening because of the CFC refrigerants is that the protective ozone layer is getting converted into incapable oxygen. Due to this the amounts of ultraviolet rays reaching the surface of the earth becomes very high and then causes excessive heating in the environment, called the greenhouse effect . The Greenhouse Effect Due to the greenhouse effect the ice in Antarctica and the poles starts getting melted, which causes large amounts of waters drifting to the rivers, causing floods. Thus, due to depletion of the ozone layer, not only the temperature increases, but also the level of water that causes floods. To prevent the depletion of the ozone layer due to CFC refrigerants, an international agreement called Montreal Protocol was signed by various countries in 1987. As per the agreement the use of the most dangerous group of CFCs comprising of R-11, R-12, R-113, R-114, R-115, R-500 and R-502, were scheduled to be phased out of production totally by January 1, 1996 in developed countries and by the year 2000 in developing countries.

slide 3 of 3 The Causes of Ozone Depletion B.C. Air Quality The Causes of Ozone Depletion Scientific evidence indicates that stratospheric ozone is being destroyed by a group of manufactured chemicals, containing chlorine and/or bromine. These chemicals are called "ozone-depleting substances" (ODS). ODS are very stable, nontoxic and environmentally safe in the lower atmosphere, which is why they became so popular in the first place. However, their very stability allows them to float up, intact, to the stratosphere. Once there, they are broken apart by the intense ultraviolet light, releasing chlorine and bromine. Chlorine and bromine demolish ozone at an alarming rate, by stripping an atom from the ozone molecule. A single molecule of chlorine can break apart thousands of molecules of ozone. What's more, ODS have a long lifetime in our atmosphere  up to several centuries. This means most of the ODS we've released over the last 80 years are still making their way to the stratosphere, where they will add to the ozone destruction.

The main ODS are chlorofluorocarbons (CFCs), hydrochlorofluorcarbons (HCFCs), carbon tetrachloride and methyl chloroform. Halons (brominated fluorocarbons) also play a large role. Their application is quite limited: they're used in specialized fire extinguishers. But the problem with halons is they can destroy up to 10 times as much ozone as CFCs can. For this reason, halons are the most serious ozone-depleting group of chemicals emitted in British Columbia. Hydrofluorocarbons (HFCs) are being developed to replace CFCs and HCFCs, for uses such as vehicle air conditioning. HFCs do not deplete ozone, but they are strong greenhouse gases. CFCs are even more powerful contributors to global climate change, though, so HFCs are still the better option until even safer substitutes are discovered. The Main Ozone-Depleting Substances (ODS) Chlorofluorocarbons (CFCs) The most widely used ODS, accounting for over 80% of total stratospheric ozone depletion.

Used as coolants in refrigerators, freezers and air conditioners in buildings and cars manufactured before 1995. Found in industrial solvents, dry-cleaning agents and hospital sterilants. Also used in foam products  such as soft-foam padding (e.g. cushions and mattresses) and rigid foam (e.g. home insulation). Halons Used in some fire extinguishers, in cases where materials and equipment would be destroyed by water or other fire extinguisher chemicals. In B.C., halons cause greater damage to the ozone layer than do CFCs from automobile air conditioners. Methyl Chloroform Used mainly in industry  for vapour degreasing, some aerosols, cold cleaning, adhesives and chemical processing. Carbon Tetrachloride Used in solvents and some fire extinguishers. Hydrofluorocarbons (HCFCs) HCFCs have become major, “transitional” substitutes for CFCs. They are much less harmful to stratospheric ozone than CFCs are. But HCFCs they still cause some ozone destruction and are potent greenhouse gases. In this section: Stratospheric Ozone Depletion - Ann M.

Middlebrook and Margaret A. Tolbert Stratospheric Ozone Depletion (Full 34-page Text in HTML) Ann M. Middlebrook National Oceanic and Atmospheric Administration Margaret A. Tolbert Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences Polar stratospheric clouds over McMurdo Station, Antarctica. The cover photograph is courtesy of David Hofmann, NOAA CMDL, Boulder, Colorado.

*Please note that the following links take you to the item's instance in the text Figures CFCs in the Troposphere as a Function of Year Tables 1) ozone produced by the photolysis of oxygen molecules into oxygen atoms 2) reaction of one oxygen atom with an oxygen molecule 3) ozone destroyed by photolysis 4) and by reaction with an oxygen atom equations 9, 10, 11 Molina and Molina ClO reacts with itself to form a dimer, (ClO)2, which in turn photolyzes into Cl atoms for ozone destruction equation 13 Solomon et al reaction on the surfaces of PSC particles, converts the stable, ozone-friendly molecules ClONO2 and HCl into a form more threatening to O3 SSAs promote the lowering of NOx Preface As the 20th century draws to a close, the global impact of human activities, once inconceivable, is now a reality. These problems must be confronted and resolved to maintain the natural balance of the planet.

In the past twenty years, the worldwide issue of ozone loss in the stratosphere has been identified, studied, and explained, and the international community has banded together to protect its mutual vital interests. During the next century, the results of this effort will be evident as the Antarctic ozone hole recedes. This module discusses the discovery of the Antarctic ozone hole, scientific studies of the processes that cause stratospheric ozone loss, and the international strategy to protect the ozone layer for future generations. Hopefully, the lessons learned from the ozone problem will pave the way for society to tackle other global and environmental issues. We dedicate this module to our children and their future. A. M. M. and M. A. T. Boulder, CO Acknowledgments This instructional module has been produced by the Global Change Instruction Program of the University Corporation for Atmospheric Research, with support from the National Science Foundation. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation. This project was supported, in part, by the National Science Foundation Opinions expressed are those of the authors and not necessarily those of the Foundation.

Introduction In the last decade, there have been numerous reports of ozone loss in the upper atmosphere. Often the newspaper headlines and sound bites are too short to provide a complete picture and are simplified to tell only one side of the story. Some claim that ozone depletion is a monumental global problem, whereas others state that ozone loss is nonexistent. Even current advertising and science fiction movies are not immune to the dramatic story of ozone loss. A recent Maybelline advertisement promotes its Natural Defense make-up by stating, "You know what's happening to the ozone, imagine what it's doing to your skin." In the movie RoboCop, there is a futuristic advertisement for a sunblock with SPF 2000 because no ozone is left to stop the Sun's ultraviolet (UV) radiation from reaching Earth. Another sci-fi movie, Highlander II, depicts a physical barrier erected over Earth to provide UV protection because the ozone layer was completely destroyed. Although the facts have sometimes been misrepresented, massive ozone loss over the Antarctic continent has been observed annually since 1979, significant ozone loss above the Arctic has been observed recently, and ozone has been eroding slowly above the United States and other populated regions. Why should we care about the ozone layer?

Quite simply, ozone naturally shields the planet from incoming UV radiation from the Sun, and this radiation destroys deoxyribonucleic acid (DNA) and proteins in all living organisms. Increased UV radiation causes increased incidence of basal cell and squamous cell skin cancer and possibly melanoma, immune system deficiencies, and cataracts in humans. In addition, the productivity of ocean phytoplankton and certain crop plants is diminished by increased UV radiation. The effects of higher UV radiation are discussed further in another module, Biological Consequences of Global Climate Change . An interesting and profound aspect of ozone depletion is that it is a global problem caused by human activities. Many people find it difficult to realize that humans can affect the environment on a global scale. In this module, we will describe the chemistry of ozone, how ozone destruction is attributed to human activity, and what society is doing to avert further damage to the ozone layer. At the end, we give a set of questions about ozone depletion. Chapter 1: The Stratosphere and the Ozone Layer On the basis of temperature, the atmosphere is subdivided into different altitude regions like the layers of an onion (see Figure 1 ). Closest to the Earth's surface is the region called the troposphere .

This region includes the air that we breathe and is where weather systems occur. The air pressure (and also the total number of molecules per unit volume) decreases almost exponentially with altitude so that roughly 90% of the mass of the atmosphere is in the troposphere and 50% of the mass is within the 5.5 km closest to the Earth. Figure 1. Variation of temperature and ozone Variation of temperature (bottom axis) and ozone (top axis) with altitude from the surface of the Earth. In the troposphere, temperature decreases with altitude. In the stratosphere, temperature increases with altitude. The tropopause is located at the minimum temperature. Ozone concentrations maximize in the stratosphere. The ozone concentration is shown on a log scale in units of the number of molecules per cubic cm (#/cm2). Adapted from Watson, R.T., M.A. Geller, R.S. Stolarski, and R F. Hampson: Present State of Knowledge of the Upper Atmosphere: An Assessment Report. NASA Reference Publication 1162, NASA, Washington, D.C., 1986, p. 20.

In the troposphere, the air is generally well mixed vertically, so many gases released on Earth tend to be mixed throughout the troposphere. We can understand the vertical mixing of the troposphere by imagining what happens when a balloon of air at the ground is raised 1 km into the atmosphere. The balloon will expand because the pressure is lower at the higher altitude and will cool because it has to do work to expand. If no heat is allowed to be exchanged between the balloon and the surrounding air, we can calculate from thermodynamics what the new balloon temperature will be. In air that contains no water, the temperature will drop 10 K per km raised. If the balloon's new temperature is higher than that of the surrounding air at that altitude, the balloon will continue to rise. This example represents unstable air that is well mixed, a condition that is very common in the troposphere. If, however the balloon's new temperature is lower than that of the surrounding air, the balloon will descend to its original position. This represents very stable air with very little vertical mixing. This situation, where cold air is below warm air, is often referred to as a temperature inversion and is a contributing factor to smog.

As seen in Figure 1 , above the troposphere the temperature begins to increase as the altitude increases. This is due to the absorption of light energy by ozone. The altitude where the lowest temperatures occur in the lower atmosphere defines the top of the troposphere and is called the tropopause ; above that is the region called the stratosphere . Because the stratosphere is a giant inversion layer with warm air above cold air, there is very slow vertical mixing in this region. In very broad terms, the global atmospheric circulation consists of upwelling into the stratosphere near the equator and downward transport of stratospheric air near the poles. The tropopause height is 16-18 km near the equator and 8-12 km near the poles. Good Ozone and Bad Ozone In Figure 1 , there is a slight increase in ozone concentrations at altitudes very close to ground level. High concentrations of ozone are often found in the troposphere in polluted areas such as Los Angeles, California. Although ozone near the ground will also absorb DNA-damaging ultraviolet radiation , it is toxic to plants and humans (see Biological Consequences of Global Climate Change ).

Therefore, ozone can be thought of as "good" in the stratosphere and "bad" in the troposphere (where we would breathe it). Consequently, efforts should be made to reduce the amount of tropospheric ozone formed by pollution and maintain the ozone layer in the stratosphere. A diagram of ozone concentration as a function of altitude is included in Figure 1 . Ozone concentrations reach a maximum in the stratosphere, forming the so-called ozone layer. Although ozone is a vital component of the stratosphere, the actual amount of ozone is very small. If all of the ozone in the atmosphere were compressed to the pressure at sea level, the ozone would form a layer only 3 mm thick, compared with 8,500 m if the entire atmosphere were similarly compressed. The chemical composition of the stratosphere is quite different from that of the troposphere. For example, water concentrations are much lower in the stratosphere because water condenses into clouds in the troposphere, and these rain out. In addition, water rising through the tropical tropopause tends to freeze out due to the extremely cold temperatures there.

Only a small amount of water vapor gets through this "cold trap." Another source of stratospheric water vapor is from the oxidation of CH4 gas (see list of chemical names and formulas ) released on Earth. Because so little water vapor is present in the stratosphere, very few clouds form there. However, these clouds play a crucial role in the ozone story. Chemicals Named in This Module Br SO2 sulfur dioxide Many of the molecules released on Earth do not reach the stratosphere because they are soluble (can dissolve) in water and return to the surface in precipitation or are broken down by chemical reactions in the troposphere. These include such molecules as HCl and NaCl. Both HCl, from erupting volcanoes, and NaCl, released into the atmosphere by ocean wavebreaking, are very soluble in water, so they are not a significant source of chlorine to the stratosphere. Some molecules, such as hydrocarbons emitted from plants and from incomplete combustion of fossil fuels, react in the troposphere with OH before they can reach the stratosphere. The only molecules that do reach the stratosphere are those that are insoluble in water and also are chemically inert (or unreactive).

A partial list of these compounds includes nitrous oxide (N2O), carbonyl sulfide (OCS), methane (CH4), chlorofluorocarbons (CFCs), and halons (carbon-based molecules containing bromine). Some of these species are natural and some are anthropogenic (generated by human activity). Although these molecules do not react in the troposphere, when they reach the stratosphere they absorb UV light and break apart. Such reactions in which molecules absorb light and fall apart are called photolysis (or photochemical) reactions. As we shall show below, photolysis of these molecules in the stratosphere affects ozone concentrations. Figure 2. Ozone Absorption Spectrum The ozone absorption spectrum showing where ozone filters UV radiation. The large absorption band of ozone between 200 and 300 nm is called the Hartley band, named after its discoverer, John Hartley. From Griggs, M.: Absorption coefficients of ozone in the ultraviolet and visible regions. Journal of Chemical Physics 49, 1968, pp. 857ï¿½ 859. ï¿½ 1968, American Institute of Physics.

Reprinted by permission. The intensity of solar radiation also varies between the troposphere and stratosphere. Both gases and particles absorb and scatter radiation, preventing the full amount of sunlight from reaching the ground. The amount of light at a given wavelength reaching the planet's surface depends on the gases and particles present in the atmosphere. A complete discussion of the Earth's radiation budget is described in the module The Sun-Earth System . Here, it is important to note that ozone absorbs UV radiation (wavelengths, , between 200 and 300 nm, see Figure 2 ), preventing high intensities at these wavelengths from reaching the ground. Table 1 shows the approximate wavelength regions of ultraviolet and visible light. If less ozone is present in the stratosphere, less UV light is absorbed there, so more reaches the ground. In fact, an increase in UV radiation has been observed as a function of decreasing ozone concentrations (see Figure 3 ). Table 1.

Ultraviolet and Visible Radiation Name CFCs and Ozone Depletion Chemistry Tutorial Ozone in the Atmosphere Ozone in the Lower Atmosphere (troposphere) About 10% of all ozone (O3) in the atmosphere is found in the troposhere (up to 16km above the earth's surface). Ozone in the troposhere has harmful effects on many living things because it is toxic. In humans, ozone causes eye irritation, compromised lung functions, aggrevation of respiratory conditions like asthma, and increases the susceptibility to infection. Ozone pollution in the troposphere is often linked to photochemical smog . Ozone in the lower atmosphere is formed during electrical discharge from high voltage appliances as shown in the equations below: O2(g) → 2O(g) O2(g) + O(g) → O3(g) Ozone in the Upper Atmosphere (stratosphere) About 90% of all ozone (O3) in the atmosphere is found in the stratosphere (16 to 32 km above the earth's surface).

In the stratosphere ozone acts as the primary UV radiation shield, short wavelength UV radiation from the sun (<240nm) provides the energy to split oxygen molecules into oxygen atoms: O2(g) Chemistry of Ozone Depletion by CFCs CFCs destroy the ozone in the stratosphere (15 - 20 km above the earth's surface) (Ozone concentrations are measure in Dobsen units, 1 Dobsen unit represents 1 molecule of O3 for every 1 billion air molecules) Ozone loss is greatest over Antarctica where the ozone depletion has been recorded and is commonly referred to as the "ozone hole". Ozone (O3), an allotrope of oxygen, is poisonous to humans if breathed in, but is important to life in that it filters out or absorbs short wavelength ultraviolet radiation (u.v) in the 280 - 320nm range which can cause serious sunburn, skin cancer and eye disorders.

The inertness and lack of water solubilty of CFCs mean they are not destroyed nor are they dissolved in rain water so they stay in the atmosphere for a very long time and diffuse up to the stratosphere In the stratosphere, CFCs come into contact with short wavelength ultraviolet radiation which is able to split off chlorine atoms from the CFC molecules CCl3F(g) These chlorine atoms destroy the ozone layer Cl(g) + O2(g) There are significant numbers of oxygen atoms in the stratosphere (since ozone undergoes a natural photochemical decomposition producing oxygen atoms and molecules) which leads to the regeneration of chlorine atoms in the stratosphere. So, 1 CFC molecule can destroy many ozone molecules. ClO(g)

Question: What component of CFCs causes destruction of ozone?
Answer: Chlorine